3Com MSR 50 Series, Configuration Manual

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5
ATM C
ONFIGURATION
Introduction to ATM
Technology
ATM Overview Asynchronous transfer mode (ATM) is a technology based on packet transmission
mode while incorporating the high speed of circuit transmission mode. It can
satisfy the need of various communication services. ATM was specified as a
broadband ISDN transmission and switching mode by ITU-T in June 1992.
Depending on its flexibility and support to multimedia services, it is regarded as
the core technology to implement broadband communications.
As defined by ITU-T, ATM transmits, multiplexes, and switches information in ATM
cells. An ATM cell has a fixed length of 53 bytes, among which 5 bytes is the cell
header and the remaining 48 bytes are payloads. The major function of the cell
header is to identify virtual connection, with limited functions on flow control,
congestion control and error control.
ATM is connection-oriented. The connection is a logical connection, or virtual
connection (VC). Each VC is identified by a pair of virtual path identifier (VPI) and
virtual channel identifier (VCI). One VPI/VCI pair has local significance only on a
segment of the link between ATM nodes. It is translated on ATM nodes. When a
connection is released, the relevant VPI/VCI pair is released and put back into the
resource table for other connections to use.
ATM interfaces support permanent virtual circuits (PVCs).
Hierarchical Structure of
ATM
The basic ATM protocol framework consists of three planes: user plane, control
plane, and management plane.
The user plane and the control plane is each subdivided into four layers, namely,
physical layer, ATM layer, ATM adaptation layer (AAL), and upper layer, each
allowing further division.
The control plane mainly uses signaling protocols to establish and release
connections.
The management plane is subdivided into layer management and plane
management. The former manages every layer in each plane and has a layered
structure corresponding to other planes. The latter is responsible for system
management and communications between different planes.
The following figure presents the relationships between layers and planes: